Processes for the vacuum withdrawal of the liquid and gaseous phases of contaminants from the ground are disclosed in the prior art. These processes are known as "two phase" vacuum extraction systems and generally employ a single vacuum generating device to remove both the liquid and gaseous phase contaminants through a single well casing. The two phase stream is then separated into gaseous and liquid streams, and the separate streams are each treated to remove contaminants. A device such as a knock out pot separates the two phases of the extracted material withdrawn by the vacuum pump. After separation, effluent in the gaseous phase is conducted through appropriate treatment steps, while the liquid phase is fed to a system such as an air stripper to remove volatile organic compounds. In some instances, the air stripper is eliminated and the liquid stream is piped directly into a sump that collects the liquid for further processing, such as pumping the liquid through a granular activated carbon (GAC) contaminant removal system.
It is also known that liquid ring vacuum pumps are useful as the source of vacuum in two phase vacuum extraction systems. Liquid ring vacuum pumps have a number of advantages over other types of vacuum generators in these applications. One advantage of liquid ring vacuum pumps is their high vacuum capability, which approaches a full atmosphere of negative pressure. Consequently, liquid ring vacuum pumps can provide the maximum possible vacuum driving force to induce the required vapor-liquid flow. Secondly, liquid ring vacuum pumps have the ability to function with a two phase flow of liquid and vapor constantly flowing through them during the operation of a two phase extraction system. Therefore, slugs of liquid or other changes in the composition of the pump influent do not adversely affect pump performance. Finally, contaminant removal in liquid ring vacuum pumps is relatively easy compared to other types of high vacuum pumps which allow lubricating oil to contact the gases passing through them. The lubricating oil can be contaminated and degraded by the hydrocarbons that are extracted from soil and groundwater at the sites where two phase vacuum extraction is used for remediation. These hydrocarbons are easily dealt with when liquid ring vacuum pumps are operated with water as a sealant. The hydrocarbons are typically only slightly water soluble and can usually be removed from the water by passing it through a granular activated carbon filter. Additional advantages of liquid ring vacuum pumps are that they are operable over a wide range of flow rates and suction resistances, they are dependable due to their small number of moving parts, and they are quiet relative to other types of high vacuum machinery.
On the other hand, a disadvantage of liquid ring vacuum pumps for application at sites undergoing two phase vacuum extraction remediation has been the requirement of a constant throughput of water to maintain the liquid ring seal essential to the pump's operation and to remove the heat of compression that would otherwise result in a temperature rise that decreases pump capacity. Supplying, treating, and disposing of this sealing water or "make up" water increases the complexity and cost of two phase vacuum extraction projects. Typically, the liquid ring vacuum pump uses a supply of sealing water from a public water system. Additionally, the prior art has disclosed that the liquid effluent from the liquid ring vacuum pump discharge vapor separator may be drawn off and added to the flow in the make up water line. Such systems thus partially recycle the water from the liquid ring vacuum pump, and decrease but do not eliminate the need for make up water. However, the higher temperature of the recycled water created by the heat of compression in the liquid ring vacuum pump reduces its performance.
Thus, the requirement of a source of make up water remains a significant drawback to the use of liquid ring vacuum pumps and limits the applicability of two phase vacuum extraction systems. In a typical vacuum extraction application, a liquid ring vacuum pump needs about 5 gallons per minute in a constant flow, and much of this flow must currently be derived from a public water system.
Another disadvantage of liquid ring vacuum pumps results from the intimate mixing and heating of the extracted soil vapors and the seal water that takes place in the pump. This mixing and heating causes the vapors to attain a very high humidity, approaching or at saturation. When the vapors leave the liquid ring vacuum pump they are separated from the water in a cyclone or other vapor/liquid separating device. However, high humidity reduces the capacity of vapor phase remediation equipment such as granular activated carbon filters that is often used to remove contaminants from the discharged vapors. Calgon Corporation, a supplier of granular activated carbon, has published a chart that indicates that the capacity of granular activated carbon to adsorb tri-chloroethene (TCE, a common contaminant removed in two phase vacuum extraction processes) is four times lower at 95% relative humidity than at 50% relative humidity. The high humidity typical of soil gases extracted with ground water in a two phase stream and the increased humidity resulting from the effects of passing those gases through a liquid ring vacuum pump therefore significantly decreases efficiency and increases the costs of treating the extracted vapors when using a humidity sensitive system such as granular activated carbon. It would therefore be desirable to dehumidify the gaseous stream exiting from the liquid ring vacuum pump. However, the efficient removal of moisture usually requires a net input of energy into the system. Since the cost of energy adds to the operating costs of the system, it would be further desirable to reduce the net amount of energy required to effect moisture removal.
Accordingly, it is an object of the present invention to provide methods and apparatus for reducing the make up water requirement in two phase vacuum extraction systems that utilize liquid ring vacuum pumps. It is a further object of the present invention to provide methods and apparatus whereby the energy required to dehumidify the gaseous effluent stream is reduced. Additionally it is an object of the present invention to provide for recovering a non-aqueous phase such as gasoline or chlorinated solvent that may be extracted from the subsurface by two phase vacuum extraction.